The invention relates to a method and an apparatus for limiting the speed of a motor vehicle to an adjustable maximum speed which separates a lower speed range to be maintained from a higher speed range to be avoided.
It is known practice in the prior art to keep the speed of a motor vehicle to a particular value, once reached, by means of a "cruise control" system. To do this, the driver releases a cruise-controller operating lever when the desired value is reached, and thus activates the cruise control system, in the process defining the instantaneous speed value as the required value. A cruise control system of this type serves, above all, to relieve the burden on the driver on long journeys when he wishes to drive at a constant speed for a prolonged period, on sections of a motorway.
German Patent document DE 37 03 645 A1 describes, for example, a cruise control system of this type in which the speed is maintained by means of a PI controller with P and I components which can be switched on and off separately. The intention in the case of the system in the German Patent document is to achieve gentle control transitions in the event of changes in the required speed value by suitable choice of the P and/or I controller component and appropriate alteration of the control parameters.
The driving situation is different when a certain maximum speed is prescribed on a section of a road. Examples which are important in practice are the speed-restricted zones in residential areas with a maximum permissible speed of 30 km/h or in other suburban areas with a maximum speed of 50 km/h. Here, the driver should be able to drive at any speed lower than the maximum speed and only the exceeding of this maximum speed should be prevented. A cruise control system is not appropriate for this purpose since the activated cruise controller always attempts to set the required speed and, furthermore, switches off automatically when the brakes are actuated. Thereafter, it has to be reactivated by accelerating to the required speed. There is therefore a requirement for systems by which the vehicle speed can be limited to an adjustable maximum speed and which, at the same time, the systems allow the vehicle to be driven at any speed below this maximum speed.
German Patent document DE 33 29 783 A1 describes measures taken on the vehicle, such as an engine-power limitation, e.g. by restricting the fuel supply, which are performed under microprocessor control if the maximum speed is exceeded in order to automatically prevent driving above a prescribed maximum speed. In a very similar manner, provision is made in the case of a device for limiting the speed of motor vehicles in accordance with German Patent document DE 29 25 566 A1 to prevent a continuous infringement of the maximum speed by rendering ineffective a--possible relatively hard--actuation of the accelerator pedal by the driver in the event of an infringement. In both systems, the maximum speed can be adjusted, either from outside or by the driver.
German Patent document DE 28 36 342 A1 describes a mechanical solution for preventing the infringement of a maximum permissible speed for a vehicle with an engine whose power can be controlled by an accelerator pedal. This solution includes the action of a positioning motor on a control element which affects the power of the engine, in particular a throttle butterfly, in such a manner that the engine power is reduced irrespective of the accelerator-pedal position as soon as the driving speed exceeds the maximum speed. The positioning motor is controlled here by a controller which compares the actual speed with the maximum speed and influences the positioning motor to an extent dependent on the difference between these speeds if the driving speed rises to within the vicinity of the maximum speed.
A vehicle-speed controller described in German Patent document DE 41 15 903 A1 can also be used to automatically prevent the infringement of a specified maximum speed. Here, the regulating measure takes effect whenever the vehicle speed exceeds the maximum speed, and includes, for example, periodic interruption of the fuel supply, brake actuation and/or reduction of the throttle valve aperture. Here too, as in the case of the above-mentioned systems that limit the maximum speed, a system intervention is thus only envisaged when the vehicle speed has reached or exceeded the specified maximum speed.
An earlier filed German Patent Application P 44 34 022.2-32, commonly owned by the assignee of the present invention, which is not a prior publication, describes a method and an apparatus for limiting the speed of a motor vehicle to a specified maximum or minimum speed, in which a selective changeover is possible between the speed-limitation function and conventional cruise control. In order to avoid conflicts between the two modes of operation in a simple manner, separate operating speed ranges for speed limitation, on the one hand, and cruise control, on the other hand, are provided there. The primary idea here is to use cruise control in the usual manner above a minimum cruise-controller required speed of about 36 km/h and to use the speed-limitation function as a maximum-speed limitation below this speed value, e.g. as a 30 km/h limitation in residential areas. In the case of the speed limitation there, a speed-controlling system intervention takes place as a function of the actual speed in accordance with a predetermined control characteristic until the actual speed is within a predetermined window of the range of speeds to be kept to, while a speed-regulating intervention takes place when the actual speed is within a predetermined speed window containing the limiting speed.
German Patent document DE 35 10 174 A1 describes a cruise-control system for maintaining a predetermined required speed which includes a speed controller and an acceleration controller arranged in parallel with one another and by which it is possible to perform, in separate time segments, either an acceleration-regulating or a speed-regulating system intervention. The selection between these two control modes takes place at the request of the driver. If the driver wishes to change the required speed to be or limited by, he chooses, on an operating unit, a corresponding operating state in which the vehicle is accelerated subject to regulation. As soon as he wishes to end the acceleration and hence maintain a constant speed, he deselects this operating state again and, as a result, the control system switches to a speed-regulating system intervention.
German Patent document DE 36 18 844 A1 describes a method for limiting the speed of an internal combustion engine, in which, above a predetermined engine-speed threshold, the required value specified by the accelerator-pedal transmitter is limited in such a way that a maximum permissible rate of change of the engine speed is not exceeded. To this end, the maximum permissible rate of change of the engine speed is set by a PI controller above the engine-speed threshold. It is possible for the maximum permissible rate of change of the engine speed and the proportional and integral components of the controller to be specified as a function of the ratio of engine speed to travelling speed. This method can also be used to limit the travelling speed to a maximum vehicle speed, for which purpose the predetermined engine-speed threshold and/or the maximum permissible rate of change of the engine speed are determined such that a particular engine speed is not exceeded. The particular engine speed corresponds to a specifiable maximum vehicle speed. In each case, limitation takes the form of a system intervention which limits the angular acceleration such that, if the driver's request is above the specified maximum permissible angular acceleration, then the angular acceleration of the engine is regulated to this maximum permissible angular acceleration and hence limited. The maximum permissible angular acceleration here falls from a maximum value determined by the dynamic response of the engine to zero as the engine speed rises to the maximum engine speed.
The underlying technical problem of the invention is to provide a method and an apparatus for limiting the speed of a motor vehicle to an adjustable maximum speed, by which method and apparatus infringement of an arbitrarily adjustable maximum-speed value is reliably prevented, while still maintaining high driving comfort.
This problem is solved by a method for limiting the speed of a motor vehicle to an adjustable maximum speed, characterized by an acceleration-regulating system intervention with a required acceleration value, specified as a function of the difference between the maximum speed and the actual speed, when the difference between the set maximum speed and the actual speed is greater than a predetermined value and the actual acceleration reaches the required acceleration value, and further characterized by a speed-regulating system intervention when the difference between the maximum speed and the actual speed is less than the predetermined value and the speed demanded by the driver is above the maximum speed. The apparatus for solving the problems includes a unit which detects the position of an accelerator pedal, a limitation function module which contains, in parallel with one another, a speed controller, an acceleration controller and a changeover switch for the selective provision of the output signal of the speed controller or the acceleration controller as the limitation output signal, and a minimum selection unit to which can be fed the output signal of the unit which detects the pedal position and the output signal of the limitation function module. The output signal of the minimum selection unit determines the actuating signal for an element which determines the driving power.
According to the method of the present invention, an acceleration-regulating intervention takes place even below a speed value which switches over the mode of operation. This value is lower than the set maximum speed. The intervention takes place as a function of the difference between the set maximum speed and the actual speed in accordance with a predetermined required acceleration characteristic. When the speed value which switches over the mode of operation is exceeded, this value typically being not very far below the maximum speed, a switch-over is then made to a speed-regulating limitation mode with the set maximum speed as the required speed. By means of the acceleration-regulating system intervention provided here, which is not described in the above-mentioned earlier German Patent Application P 44 34 022.2-32 and which takes place significantly below the set maximum speed, a smooth approach of the actual speed to the maximum speed with a speed of approach very largely independent of the maximum speed and of the instantaneous engine load is achieved. This is true without the actual speed overshooting the maximum speed even in the case of relatively high set maximum speeds.
As a further significant characteristic of the method, no system intervention takes place while the speed demanded by the driver remains below the maximum speed, i.e. until the actual acceleration is above the acceleration value specified for the respective actual speed by the required acceleration characteristic. This ensures that, below the set maximum speed, the motor vehicle can be driven at any desired speed in accordance with the demands of the driver, without being influenced by the system. In contrast to conventional cruise control systems, the speed limitation method remains in readiness even when the driver brakes the vehicle from an actual speed above or equal to the maximum speed to a speed significantly below the actual speed. In the event of a subsequent attempt to accelerate, the limitation method then prevents a maximum-speed infringement, initially by the acceleration-regulating system intervention and then by the speed-regulating system intervention.
An advantageous apparatus is provided according to the present invention for carrying out the above method. For carrying out the acceleration-regulating and speed-regulating phases, an acceleration controller and a speed controller are provided in parallel, their output signals being fed to a change-over switch which can be driven by the speed error. The minimum selection unit on the output side ensures that speed adjustment takes place in accordance with the demands of the driver unless there is a risk that the set maximum speed will be exceeded. Only when this is the case, e.g. due to a demand from the driver for a high acceleration, does the minimum selection unit allow one of the controller output signals through instead of the accelerator-pedal signal as the actuating signal for a control element which determines the driving power, e.g. a throttle butterfly of an Otto engine or an injection-quantity control element of a diesel engine.
A further preferred embodiment of the present invention provides speed-controlling system interventions in certain special situations, so that even in these situations a gentle, jerk-free approach of the actual speed to the set maximum speed or, in the case where the limitation function has been switched off, to the speed demanded by the driver is achieved.
In order to reliably be able to keep to the set maximum speed in the limitation function even when travelling down grade, transmission downshifts are additionally provided as a advantageous further development of the present invention.
As a further preferred embodiment of the present invention, a permanent limitation function is provided in addition to the activatable and deactivatable limitation function with a maximum speed value that can be freely adjusted by the driver. This permanent limitation function remains active at all times during the journey and limits the speed to a maximum speed value which is set once, for example in the workshop, and which the vehicle is not to exceed at any time. A permanent limitation function of this type is appropriate, for example, when the vehicle is fitted with winter tires and should only be driven up to a maximum speed, determined by the tires, which is below the maximum speed that can be achieved by the vehicle. Another application is with vehicles which are used in countries with a legally stipulated speed limit.
As an advantageous embodiment of the invention, the apparatus which performs the method additionally has a cruise controller of conventional design in which either only cruise control or only the variable speed limitation is activated. If permanent maximum-speed limitation is provided, this remains active even when the cruise control has been activated. The required speed values which can be set for cruise control are limited to a range which maintains a predetermined distance from the permanent maximum-speed value in order to avoid conflicts between the permanent limitation function and the cruise control.
A preferred embodiment of the invention includes convenient, user-friendly operation by means of a single operating lever for the cruise-control function and the speed-limitation function with the possibility of setting maximum-speed values and of individual programming of a plurality of such values.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.